1. Technical Field
This invention relates to signal communication systems and, more particularly, to the reduction of quantization noise in digital signal compression.
2. Related Art
One prevalent mode of human communication uses communication systems. Communication systems include both wireline and wireless radio based systems. Wireless communication systems are electrically connected with wireline based systems to form a communication network. One form of communication within the communication network is digital transmission of signals such as speech, audio etc. Digital transmission may involve the steps of: sampling an analog waveform with an analog-to-digital converter, compression (encoding), transmission, decompression (decoding) and digital-to-analog conversion to synthesize the analog waveform. Compression involves minimizing the number of bits used to encode the signal in an effort to maximize the available bandwidth within the communication systems.
One coding technique involves the use of quantizers that perform sample-by-sample quantization of the signal. The signal may be divided into discrete samples to facilitate encoding and decoding. The samples may be further processed to obtain parameter values representing the samples. In general, quantization involves using a table of quantization levels that are selectively chosen to represent the parameter values. Selection of the quantization levels is based on minimizing the error between the parameter values and the quantization levels during a series of quantizations. Sample-by-sample quantization typically occurs in an encoder in order to compress the signal for transmission. Following transmission, the decoder uses the same table of quantization levels to reconstruct the signal. The encoding and decoding of a signal may occur many times within the communication systems.
A known problem in sample-by-sample quantization is the introduction of audible distortion known as quantization distortion at “low” bits-rates. A signal that is compressed with A-law or μ-law as described in the digital speech coding standard G.711 is one example of sample-by-sample quantization that introduces quantization distortion into the signal. Consequently, a signal that has been encoded and decoded with G.711 may contain undesirable audible quantization noise, thereby reducing the perceptual quality. Accordingly, a need exists for systems of reducing the quantization distortion introduced during sample-by-sample quantization.